Anodized aluminum



United States Patent 3,175,963 ANGBIZED ALUMINUM Saul Kessler, Liberty Lake, Wash, assignor to Kaiser Aluminum 8; Chemical Corporation, (lakiand, Calih, a corporation of Delaware N0 Drawing. Filed Jan. 28, 1963, Ser. No. 254,494 14 Claims. (Cl. 204--35) This invention relates to an anodic oxide coating on an aluminum body and the process or method of forming said coating. Particularly, this invention relates to an anodic oxide coating on an aluminum body that is resistant to thermally induced crazing.

Crazing is a condition characterized by the formation of fine, hairline cracks in a coating on a basis material, which are sometimes so fine that they are visible only with oblique lighting. Crazing may be caused in a anodic oxide coating on an aluminum body by subjecting the body to high temperatures. In certain instances anodized aluminum may be included in an assembly that requires a high temperature treatment. For example, anodized aluminum used for automobile trim may be partly covered with accent paint which must be baked to place the paint in its desired ultimate condition. The temperature required to bake the paint may be sufliciently high to cause the anodic coating on the aluminum body to craze. Crazing is not only unsightly, but it adversely aliects the corrosion resistance of the oxide coated body.

It is an object of this invention to provide an anodized aluminum body that is resistant to crazing at high temperature and a method for producing the same.

In the specification and appended claims, the terms anodic oxide coating or film, which are common terms in the art, means an oxide coating on an aluminum body that is formed while electric current is passing through the aluminum body and while it is the anode in an electrolytic bath. The term anodized aluminum means an aluminum body having an anodic oxide coating on its surface, the term anodizing means the process of forming an anodic oxide coating; and the term aluminum body means any solid metal body that is predominately aluminum.

In the known process of anodizing aluminum, an aluminum body is placed in a bath of sulfuric acid or other suitable electrolyte and connected as the anode in a directcurrent electric circuit which includes the electrolyte bath. When current is passed through the bath, an oxide layer is formed on the aluminum body that is characterized by being thicker than an oxide layer that would form in air.

An anodic oxide coating, as formed, is porous and can be penetrated by staining substances. To reduce porosity, and to improve chemical stability, the anodic oxide coating is usually sealed by immersing it in hot water. The sealed, anodized surface is decorative, resistant to corrosion and resistant to staining.

The method of the present invention includes the use of para-toluenesulfonamide, sodium lignosulfonate or both as coating-modifying agents dissolved in an aqueous medium employed in the production of an anodic oxide film or coating on an aluminum body. The coatingmodifying agent may be dissolved in the electrolyte in which the anodic oxide is formed, it may be dissolved in the sealing bath, it may be dissolved in both the electrolyte and the sealing bath or even in a separate bath employed only for subjecting anodic oxide coatings to contact with coating-modifying agents.

The presence of any para-toluenesulfonamide or sodium lignosulfonate will improve the crazing resistance of anodic oxide, but for significant improvement, an effective concentration must be present. Para-toluenesulfonamide may be used in concentrations up to the limit of its solubility, and at least 0.005 gram per liter and pref- "ice erably from about 0.01 to about 0.05 gram per liter of para-toluenesulfonamide should be used. At least about 0.005 and up to about 5.0 grams per liter or more and preferably from about 0.1 to about 3.0 grams per liter of sodium lignosulfonate should be used. Although it is not known by what mechanism the coating-modifying agent changes the character of an anodic oxide film, it is thought that it becomes part of the film by physical or chemical combination with it. However, as will be demonstrated in the following examples, the presence of paratoluenesulfonamide, sodium lignosulfonate, or preferably both of these coating-modifying agents in an aqueous medium employed in making anodized aluminum causes significant increases in the temperature to which the anodized aluminum body may be subjected before crazing occurs.

Example I An anodic oxide coating was produced on 24 panels of an aluminum alloy sheet having the Aluminum Association designation 5457 and consisting of aluminum containing a maximum of 0.08% silicon, 0.20% copper, 0.15 %0.45 manganese, 0.10% iron, 0.80%1.20% magnesium and 0.10% other material. Each panel was four inches by six inches in size and the anodic oxide coating was produced by connecting each panel as the anode in an electric circuit while submerged in an electrolyte consisting of 15% by weight sulfuric acid in water. The electrolyte was maintained at 70 F. and anodizing was efieoted at a current density of 12 amperes per square foot until a coating approximately 0.3 mil in thickness was produced. In each of the diiierent sealing baths described below six panels were immersed for 10 minutes at a temperature of 210 F.

A. Pure de-ionized water.

B. De-ionized Water containing 0.01 gram per liter of para-toluenesulfonamide dissolved therein.

C. De-ionized water containing 0.03 gram per liter of para-toluenesulfonamide dissolved therein.

D. De-ionized water containing 0.05 gram per liter of para-toluenesulfonamide dissolved therein.

The panels were then heated to crazing temperature in the following manner. The panels were placed between two 250 watt infrared bulbs with one bulb on either side of the panel two inches from its surface. The panels were heated in this manner to a temperature well below crazing temperature, cooled and inspected, and then reheated to a temperature 10 higher. The heating to suecessively higher temperature was continued until approximately 15 of the surface was crazed, and the last temperature reached was recorded as the crazing temperature. The crazing temperature of the panels seated in pure water averaged 318 F., the crazing temperature of the panels sealed in solution B averaged 325 F., the crazing temperature of the panels sealed in solution C averaged 412 F., and the crazing temperature of the panels sealed in solution D averaged 398 F. The use of para-toluenesulfonamide, therefore, increased the crazing temperature at all concentrations, the increases ranging from 7 F. to 94 F.

Example 2 Another series of 40 panels were anodized in the same manner as those of Example 1 except that each group of eight panels were anodized to have coatings of different thicknesses. Twenty of the panels, four from each group of eight, were sealed in pure de-ionized water and the other 20 were sealed in water containing 0.03 gram per liter of para-toluenesulfonamide. Sealing was effected by immersing the anodized panels for 10 minutes in the indicated solutions which were maintained at 210 F.

The panels were then heated as set forth in Example 1 to determine the crazing temperature. The following table sets forth the average results obtained.

This example illustrates that anodized coatings of all thicknesses have their crazing temperature increased by the use of this invention, although some coatings are improved to a different extent than others.

Example 3 Four panels were prepared with anodic oxide coatings 0.3 mil thick by treating them as set forth in Example 1 except that the electrolyte solution in one cell contained para-toluenesulfonamide in a concentration of 0.005 gram per liter. After anodizing, the panels were sealed by immersing them in boiling de-ionized water for 10 minutes. The panels were tested to determine their minimum crazing temperature by placing them in a Calrodheated oven on a rotating holder so that all portions of the panels received equal heating and by observing the surface of the panels through a window and recording the temperature at which the first hair-line crack in the anodic oxide coating appeared. The panels that were anodized in an electrolyte containing para-toluenesulfonamide showed the first craze line at an average temperature of 370 F. while the panels that were anodized in the absence of para-toluenesulfonamide showed the first craze line at an average temperature of 320 F.

Example 4 Four panels were anodized as described in Example 1 to a thickness of 0.3 mil and sealed by immersing them for 10 minutes in a boiling solution of 1 gram per liter of sodium lignosulfonate in water. The commercial product marketed by Crown-Zellerbach Company under the trade name Orzan-S was employed as a source of sodium lignosulfonate. The panels were dried and heated in the Calrod-heated oven as described in Example 3. The average crazing temperature for the panels was 431 F.

Example 5 Four panels were anodized as described in Example 1 to a thickness of 0.3 mil and sealed by immersing them for minutes in a boiling solution of 1.0 gram of sodium lignosulfonate and 0.03 gram of para-toluenesulfonamide per liter of water. The panels were dried and heated in the Calrod-heated oven as described in Example 3. The average crazing temperature for the panels was 456 F.

Example 5 illustrates that a synergistic effect is obtained by the use of both para-toluenesulfonamide and sodium lignosulfonate in treating anodic oxide coatings in that a remarkably high temperature is required to cause crazing of a coating sealed in the presence of both. It has also been observed that anodic oxide coatings formed in the presence of sodium lignosulfonate are free from smut, a loose surface layer that frequently is formed on water-sealed anodic oxide coatings. It is disturbed by any handling so that unsightly white smears are formed.

Generally speaking, thin anodic oxide coatings resist crazing better than thick coatings. However, the coating must be thick enough to protect the basis metal under the conditions of use. Anodized aluminum used for automotive trim should have an anodic oxide coating between races about 0.2 and 0.4 mil thick. Architectural anodized aluminum, whether colored or uncolored, and whether used as extrusions or sheets, if used outdoors and unmaintained should have a coating or from about 0.7 to 2.0 mils while architectural aluminum used indoors or maintained in outdoor use should be between about 0.4 and 1.0 mil thick. Especially for the very thick films, the process of this invention is useful to prevent crazing during sealing.

It has also been found that the presence of any sodium lignosulfonate or para-toluenesulfonamide in one or both of the aqueous solutions employed in making sealed anodized coatings will provide increased resistance to crazing. Significant improvements in crazing resistance generally require that at least one aqueous solution contain at least about 0.005 gram per liter of para-toluenesulfonamide or sodium lignosulfonate. Generally it has been found that sealing solutions containing from about 0.02 to about 0.04 gram per liter of para-toluenesulfonamide and from about 0.1 to about 3.0 grams per liter of sodium lignosulfonate produce the most satisfactory crazing resistance for coatings in the preferred thicknes s xvf' ranges while electrolyte solutions should contain at least about 0.005 gram per liter of either or both coating-modifying agent.

It has also been found that prolonging the sealing period or the time during which the anodized aluminum body is immersed in the sealing bath diminishes the crazing resistance of the ultimate sealed coating. Sealing must be continued long enough to create a chemically stable coating but not so lon as to destroy the resistance to thermal crazing that is imparted by the coating-modifying solutions. It has been found that coatings are adequately sealed and quite resistant to crazing when immersed in boiling sealing solutions for a period of from about 5 to about 15 minutes, and preferably for about 10 minutes.

The tests reported in the examples should not be construed as determining absolute values for crazing temperatures of the coatings involved or those formed on other aluminum compositions. The tests employed were to obtain comparative values only, and the panels were subjected to conditions more severe than would normally be encountered in use. Some panels used in the tests were heated rapidly so that gradual differential expansion could not take place, others were held at high temperatures for long periods and other panels were repeatedly heated and cooled. It is contemplated that for some uses, temperatures substantially higher than the crazing temperatures reported can be endured, especially for short periods, without crazing of an anodic oxide coating. In other words, the examples are comparative, and indicate only the degree of improvement efiected by this invention at a given set of conditions, rather than indieating a limit to the operability of the invention.

What is claimed is:

l. The process of producing a crazing-resistant anodic oxide coating on an aluminum body which comprises immersing said body in an aqueous electrolyte bath while it is connected as an anode in an electric circuit which includes said electrolyte bath, passing electric current through said body to produce an anodic oxide coating on said body, removing said body from said electrolyte bath and immersing it in a hot aqueous sealing bath with at least one of said aqueous electrolyte bath and said aqueous sealing bath containing para-toluenesulfonamide. I

2. The process of claim 1 further characterized in that said electrolyte bath comprises dilute sulfuric acid.

3. The process of claim 1 further characterized in that said electric current is passed through said body at a current density of from about 5 to about amperes per square foot for a period sulficient to produce an anodic oxide coating having a thickness of from about 0.2 to about 0.4 mil for use as automotive trim.

4. The process of claim 1 further characterized in that said electric current is passed through said body at a current density of from about 5 to about 120 amperes per square foot for a period sufiicient to produce an anodic oxide coating from about 0.4 to about 1.0 mil thick for indoor architectural use.

5. The process of claim 1 further characterized in that said electric current is passed through said body at a current density of from about 5 to about 120 amperes per square foot for a period suilicient to produce an anodic oxide coating from about 0.7 to about 2.0 mils thick for outdoor architectural use.

6. The process of producing a crazing-resistant anodic oxide coating on an aluminum body which comprises immersing said body in an aqueous electrolyte bath while it is connected as the anode in an electric circuit which includes said electrolyte bath, passing electric current through said body to produce an anodic oxide coating on said body, removing said body from said electrolyte bath and immersing it in a hot aqueous sealing bath containing para-toluenesulfonamide.

7. The process of claim 6 further characterized in that para-toluenesulfonamide is present in said sealing bath in a concentration of from about 0.02 to about 0.04 gram per liter.

8. The process of claim 6 further characterized in that scaling is effected by immersing said aluminum body in said sealing bath for a period of from about 5 to about minutes.

9. The process of claim 6 further characterized in that said sealing bath consists essentially of para-toluenesulfonamide and sodium lignosulfonate in water.

10. The process of producing a crazing-resistant anodic oxide coating on an aluminum body which comprises immersing said body in an aqueous electrolyte bath having para-toluenesulfonamide dissolved therein while said body is connected as the anode in an electric circuit including said electrolyte bath, and passing electric current through said body to produce an anodic oxide coating on said body.

11. The process of claim 10 further characterized in that said electrolyte bath contains para-toluenesulfonamide in a concentration of at least about 0.005 gram per litre.

12. The process of producing a crazing-resistant anodic oxide coating on an aluminum body which comprises immersing said body in an aqueous electrolyte bath having para-toluenesulfonamide dissolved herein, while said body is connected as the anode in an electric circuit which includes said electrolyte bath, passing electric current through said body to produce an anodic oxide coating on said body, and removing said body from said electrolyte bath and immersing it in a hot aqueous sealing bath con taining para-toluenesulfonamide.

13. The process of sealing an anodized aluminum body which comprises immersing said body in a hot aqueous sealing bath containing para-toluenesulfonamide.

14. An anodized aluminum body that is resistant to crazing at high temperatures prepared by the process which comprises immersing an aluminum body in an aqueous electrolyte bath while it is connected as an anode in an electric circuit which includes said electrolyte bath, passing electric current through said body to produce an anodic oxide coating on said body, removing said body from said electrolyte bath and immersing it in a hot aqueous sealing bath with at least one of said aqueous electrolyte bath and said aqueous sealing bath containing para-toluenesulfonamide.

References Cited in the file of this patent UNITED STATES PATENTS 2,888,388 Stiller May 26, 1959 FOREIGN PATENTS 636,293 Great Britain Apr. 26, 1950 

1. THE PROCESS OF PRODUCING A CRAZING-RESISTANT ANODIC OXIDE COATING ON AN ALUMINUM BODY WHICH COMPRISES IMMERSING SAID BODY IN AN AQUEOUS ELECTROLYTE BATH WHILE IT IS CONNECTED AS AN ANODE IN AN ELECTRIC CIRCUIT WHICH INCLUDES SAID ELECTROLYTE BATH, PASSING ELECTRIC CURRENT THROUGH SAID BODY TO PRODUCE AN ANODIC OXIDE COATING ON SAID BODY, REMOVING SAID BODY FROM SAID ELECTROLYTE BATH AND IMMERSING IT IN A HOT AQUEOUS SEALING BATH WITH AT LEAST ONE OF SAID AQUEOUS ELECTROLYTE BATH AND SAID AQUEOUS SEALING BATH CONTAINING PARA-TOLUENESULFONAMIDE. 